Two months after clinching his fifth Nutri-Grain Ironman title victory, the Kiama superstar was in dominant form at the Australian Surf Life Saving Championships on Wednesday on the Sunshine Coast.

Day claimed victory in his heat in the open ironman category from Finn Askew and Dan Collins to advance to the event's quarter-finals on Thursday.

The 33-year-old said post-race that it was "good to blow the cobwebs out".

"It's been completely different to anything we've raced in this year. It's hard in different ways, you've got to be switched on with your skills," Day said.

"But it was really good to get through. Anything can happen in these conditions, so I'm grateful to have my foot on the start line (on Thursday). It's about taking it race by round, round by round, day by day."

The Australian SLSC Championship is Day's first major event since claiming his fifth national Ironman title in February.

The 2024 crown came 10 years after he won it for the first time.

"I don't know if it will ever sink in... I won it at 23 for the first time and I am 33 now. Five is an awesome number," Day said after the Ironman victory.

"I wanted to prove to myself I could back up and do another one. It just feels so good."